1. Field of the Invention
The present invention relates to helical antennas, and, more particularly, to an apparatus and method which tilts closely spaced helical radiators in a multiple element array to increase the effective aperture of the antenna.
2. Description of the Related Art
Helical antennas generally consist of a single conductor or multiple conductors (multi-filar) wound into a helical shape. Although such antennas can radiate in many modes, the axial mode is most commonly used. The axial mode generates maximum radiation along the helix axis. By constructing the helix such that the helix's circumference is on the order of one wavelength of the radiation being emitted, the helix will radiate in the axial mode. The radiation emitted from a helical antenna is circularly polarized with the handedness of the polarization determined by the handedness in which the helix is wound.
FIG. 11 graphically depicts the substantially overlapping apertures (700,710) of a first helical radiator 720 and second helical radiator 730. The first and second helical radiators (720,730) have first and second radiation apertures (700,710), respectively. The first radiation aperture 700 and the second radiation aperture 710 substantially overlap to form an aperture overlap region 740.
The first helical radiator 720 is spaced apart from the second helical radiator 730 by a radiator spacing L. As the radiator spacing L decreases from .lambda., the aperture overlap region 740 correspondingly increases in size. This increase in the aperture overlap region 740 decreases the effective aperture of the antenna array. The effective aperture is the union of the first radiation aperture 700 and second radiation aperture 710.
FIG. 12 shows a conventional two helical element antenna array having a first helical radiator 820 and second helical radiator 830. The first and second helical radiators (820, 830) have first and second radiation apertures (800,810).
By spacing the first helical radiator 820 apart from the second helical radiator 830 by a radiator spacing L' that is approximately a wavelength .lambda., the aperture overlap region 740 (shown in FIG. 11) may be avoided. Thus, the effective aperture of the antenna array shown in FIG. 12 is at a maximum because the aperture overlap region does not occur. Such a large radiator spacing L', however, increases the size of the antenna array. Because compact antenna arrays are advantageous, such an increase in size should be avoided.
When compared with a single helical element antenna, the dual helical element antenna shown in FIG. 12 has an effective aperture and gain which is twice (3 dB) that of the single element helical antenna. As the spacing L' is reduced, the effective aperture of the two helical element antenna quickly approaches the effective aperture of a single helical antenna. When the two helical radiators 820, 830 are spaced apart by approximately one wavelength of the signal to be radiated, however, the size of the antenna increases. Such size increases are undesirable.